Two types of tyrosine hydroxylase (TH)-immunoreactive neuron are found in the rhesus monkey, Macaca mulatta, retina. One of these types is immunoreactive for the presence of aromatic-L-amino acid decarboxylase (AADC), and the other is not. These results suggest that the TH-containing, non-AADC immunoreactive cell type does not process L-dopa to dopamine. It is intended to further investigate this cell type to determine if it contains an endogenous catecholamine or fluorescing amino acid such as L-dopa. Such evidence at face value suggests that L-dopa, the precursor of dopamine may be an endogenous neurotransmitter in the CNS. The morphology, localization and time of appearance of amino-acid transmitters, particularly GABA are being studied in the embryonic- murine brain and spinal cord. GABA-containing neurons may first appear as early as embryonic day (E)-11 and are certainly prominent in the rhombencephalon by E-12 before their appearance more rostrally in the mesencephalon and prosencephalon, and their appearance more caudally in the spinal cord. By E-13, GABA- immunoreactive neurons are present throughout the embryonic CNS and subsequently proliferate rapidly.